1. Field of the Invention
The present invention relates in general to methods of producing a piston of an internal combustion engine and more particularly to methods of applying an outer surface of a skirt portion of the piston with a double layer solid lubricant film.
2. Description of Related Art
One of the methods for applying the skirt portion of a piston with such double layer solid lubricant film is shown in Japanese Laid-open Patent Application (tokkai) 2010-216362.
In the method, a machined or naked piston is subjected to a water washing to be cleaned and then the cleaned piston is dried. Then, a composition for an inner layer film, which shows a less wearing property, is applied through a screen printing to an outer surface of a skirt portion of the naked piston and fired in a certain condition to form a first solid lubricant film (viz., the inner layer film). Then, the piston having the first solid lubricant film just fired is cooled and a composition for an outer layer film, which shows a higher wearing property, is applied through a screen printing to the outer surface of the first solid lubricant film and fired in a certain condition to form a second solid lubricant film (viz., the outer layer film) on the first solid lubricant film. With this processing, a double layer solid lubricant film is produced on the outer surface of the skirt portion of the piston.
With the above-mentioned method, streaks left on the outer surface of the skirt portion of the piston are effectively covered by the double layer solid lubricant film. Under operation of the piston thus produced in a corresponding engine cylinder, the second solid lubricant film is worn relatively early due to its higher wearing property and the first solid lubricant film, which has the less wearing property, is then exposed to the outside and slowly worn but smoothly. With such smoothed outer surface, the piston can exhibit a very low friction when operates in the engine cylinder.